Negative electrode material for power storage device, electrode structure, power storage device, and production method for each

ABSTRACT

Provided is an anode active material for energy storage devices capable of electrochemically inserting and extracting lithium ions and production method thereof, an electrode structure including the active material and flake graphite, and an energy storage device using the electrode structure as an anode. The anode active material includes secondary particles that are aggregates of 10-300 nm primary particles containing silicon as a main component. The primary particles each include, as a surface layer, a composite metal oxide layer containing at least one or more metal elements selected from at least Al, Zr, Mg, Ca, and La and Li.

TECHNICAL FIELD

The present invention relates to a method for producing an anodematerial that contains silicon, which forms an alloy with lithium in anelectrochemical reaction, as a main component and can store and releaselithium ions, a method for producing an electrode structure formed ofthe material, and a method for producing an energy storage deviceincluding the electrode structure.

BACKGROUND ART

The possibility that the green house effect due mainly to increases inthe amount of CO₂ in the atmosphere may be changing the climate of theearth has been pointed out in recent years. It has been also pointed outthat the air pollutants, including CO₂, NO_(x), and hydrocarbons,discharged from automobiles being used as moving means affect health. Interms of the protection against increases in the prices of energyresources such as oil, the preservation of the environment, or theprotection against disasters, high expectations have been recentlyplaced on hybrid vehicles, which are each a combination of an electricmotor driven by electricity stored in a storage device and an engine andwhich are energy-efficient, electric vehicles, smart grids, which aresystems for managing energy from energy plants through a network andoptimizing the demand/supply balance of energy, and energy storagesystems. In the field of information communication, informationterminals such as smartphones are rapidly penetrating the society, sincethey easily receive and transmit information. Under these circumstances,energy storage devices, such as capacitors or rechargeable batteries,are required to have high output density, high energy density, and longlife to improve the performance of smartphones, hybrid vehicles,electric vehicles, smart grids, and the like, as well as to reduceproduction cost.

Among the commercialized energy storage devices, lithium-ionrechargeable batteries (commonly called “lithium-ion batteries”), whicheach use carbon, such as graphite, as an anode and a compound of lithiumand transition metals as a cathode, have the highest energy density.However, a lithium-ion rechargeable battery, whose anode is formed of acarbon material, can theoretically intercalate only a number of lithiumatoms corresponding to up to ⅙ the number of carbon atoms. For thisreason, it is difficult to further increase the capacity, and increasingthe capacity requires a new electrode material. Lithium-ion rechargeablebatteries are expected to become power supplies for hybrid vehicles orelectric vehicles thanks to their high energy density. On the otherhand, lithium-ion rechargeable batteries have a problem that whenquickly discharged, they cannot discharge a sufficient amount ofelectricity due to their large internal resistance, that is, theiroutput density is low. For this reason, there is a demand to develophigh-output-density, high-energy-density energy storage devices. Tosatisfy this demand, tin, silicon, and alloys thereof, which can storeand release more lithium ions than graphite, are being studied. Tin andsilicon can electrochemically store more lithium ions. However, theseelements expand their volumes by a factor of as many as about four andrepeatedly expand and contract through charges and discharges, resultingin increases in the electrode resistance and reductions in the electrodeperformance. To prevent reductions in the electrode performance, therehave been made various proposals about silicon particles themselves,conductive additives, binders, and current collector substrates.

Non-Patent Literature 1 proposes, as an electrode having longcharge-discharge cycle life, a silicon nanotube that contains expansionspace and whose outermost surface is coated with silicon oxide.Non-Patent Literature 2 proposes a silicon nanowire which is coated withalumina by atomic layer deposition, as an electrode active materialhaving long cycle life. However, the silicon nanotube needs to beproduced through many steps and therefore is not suitable for massproduction, nor is the silicon nanowire suitable for mass production.Accordingly, any of the silicon nanotube and silicon nanowire isdifficult to provide cheaply. Nor is the coating with alumina suitablefor mass production.

To suppress the crack of particles during lithiation (lithiuminsertion), attempts are being made to produce an anode for rechargeablebatteries that includes submicron or less sized silicon particles(commonly called “silicon nanoparticles”), which are easilymass-produced unlike the silicon nanotube and silicon wire. However,even such silicon particles have low electron conductivity and expandduring lithiation as well. For this reason, Non-Patent Literature 3 andthe like propose forming a composite with graphene to improve electronconductivity and to ensure expansion space. However, a method forproducing graphene used to improve conductivity described in Non-PatentLiterature 3 is not suitable for mass production. On the other hand,Patent Literature 1 proposes a composite of a battery active materialcapable of forming an alloy with lithium, such as silicon, and expandedgraphite or flake graphite. However, any of expanded graphite and flakegraphite has a large particle size and therefore it is difficult to mixand disperse and submicron or less-sized silicon particles uniformly.Non-Patent Literature 4 proposes forming a composite of a graphitenanosheet and silicon particles by immersing expanded graphite in atetrahydrofuran solution of silicon nanoparticles and polyvinyl chlorideand applying ultrasound to the expanded graphite to flake off theexpanded graphite. However, a flake-off device using ultrasound isdifficult to be scaled up and therefore is not suitable for massproduction. For this reason, there is a demand to develop a method forproducing an easy-to-mass-produce, cheap conductive additive that issuitable for fine silicon particles.

Patent Literature 2 proposes metal coating for improving the electronconductivity of silicon particles and ceramic coating for suppressingthe pulverization of silicon particles caused by expansion duringlithiation. Patent Literature 3 proposes means of extending thecharge-discharge cycle life by providing silicon particles with a metaloxide coating layer formed of a raw material, such as alkoxide, bysol-gel reaction. Patent Literature 4 proposes silicon nanoparticlescoated with a metal oxide coating and dispersed in silicon oxide inorder to suppress gas generated by the decomposition of an organicsolvent in an electrolyte solution. In any of the proposals, the coatingof silicon particles does not reduce the amount of silicon oxide thatcauses an irreversible lithiation-delithiation capacity duringcharge-discharge but rather often increases the amount, resulting in areduction in the initial charge-discharge coulombic efficiency.

To form the anode of a lithium-ion battery using silicon particles as anactive material, it is important that the binder be formed of a materialcapable of enduring the expansion and contraction of the volume duringlithium storing/releasing. As the material of the binder, Non-PatentLiterature 5 proposes carboxymethylcellulose; Non-Patent Literature 6proposes sodium alginate; and Non-Patent Literature 7 proposespolyacrylic acid. However, these materials have a problem that any ofthe polymers thereof does not provide sufficient strength when used in asmall amount and reduces the conductivity of the electrode when used inan amount to maintain strength. For this reason, there is a demand todevelop a technique that can increase the mechanical strength of theelectrode even when a small amount of binder is used.

An electrode in which an electrode layer including silicon particles areformed on a current collector formed of a metal foil expands orcontracts due to the storing or releasing of lithium. The currentcollector receives stress accordingly. Since the electrode layer is notcompletely uniform, the metal foil as a current collector becomescrinkled ununiformly, thereby flaking off the electrode layer. Thisincreases the electrical resistance of the electrode and reduces thecharge-discharge cycle life. For this reason, there is a demand for acurrent collector or electrode structure that does not become crinkledununiformly. Non-Patent Literature 8 discloses that stress caused by thelithiation is released by using a flexible current collector formed byevaporating a metal thin-film onto a flexible substrate. However, themetal thin-film disadvantageously cannot cope with the charge ordischarge of large current. In order to suppress the distortion orbreakage of the anode current collector and thus the deformation of theanode, Patent Literature 5 proposes an electrode including a currentcollector having thereon multiple protrusions and an alloy-based activematerial disposed on the protrusions. However, this electrode has aproblem that a step of eliminating the distortion of the currentcollector is required and a problem that the lamination of the activematerial only on the protrusions increases the number of productionsteps, as well as the production cost.

To improve the cycle life of an electrode using silicon particles as anactive material, Non-Patent Literature 9 proposes an electrolytesolution containing vinylene carbonate as an additive; Non-PatentLiterature 10 proposes an electrolyte solution containing fluoroethylenecarbonate as an additive. These electrolyte solutions extend the lifecompared to those not containing these additives. However, the thicknessof a solid electrolyte interphase (SEI) layer generated by thedecomposition of the electrolyte solution is moderately increased, andthe conductivity of the electrode is reduced. That is, the growth of anSEI layer is not sufficiently suppressed. For this reason, there is ademand for an additive which is effective in suppressing the growth ofan SEI layer.

CITATION LIST Patent Literature

[Patent Literature 1] Japanese Unexamined Patent Application PublicationNo. 2013-219018

[Patent Literature 2] Japanese Unexamined Patent Application PublicationNo. 2006-19309

[Patent Literature 3] Japanese Unexamined Patent Application PublicationNo. 2006-190642

[Patent Literature 4] Japanese Unexamined Patent Application PublicationNo. 2011-96455

[Patent Literature 5] WO2011/093015

Non-Patent Literature

[Non-Patent Literature 1] Nature Nanotechnology, 7, 310-315 (2012).

[Non-Patent Literature 2] The Journal of Materials Chemistry, 22,24618-24626 (2012).

[Non-Patent Literature 3] The Journal of Physical Chemistry Letters, 3(13), 1824-1829 (2012).

[Non-Patent Literature 4] Carbon, 72, 38-46 (2014).

[Non-Patent Literature 5] Chemistry of Materials, 22 (3), 1229-1241(2010).

[Non-Patent Literature 6] Science, 334, 75-79 (2011).

[Non-Patent Literature 7] ACS Applied Materials & Interfaces, 2 (11),3004-3010 (2010).

[Non-Patent Literature 8] Advanced Energy Materials, 2, 68-73 (2012).

[Non-Patent Literature 9] Journal of Power Sources, 174, 538-543 (2007).

[Non-Patent Literature 10] Chemical Communications, 48, 7268-7270(2012).

SUMMARY OF INVENTION Technical Problem

In view of the foregoing, an object of the present invention is toprovide an anode active material (anode material) that canelectrochemically store and release a large amount of lithium ions andthat is formed of a material having a high ratio of the initiallithium-ion release amount to the initial lithium-ion storage amount andcontaining silicon as a main component, and production method thereof.

Another object of the present invention is to provide a long-lifeelectrode structure formed of an active material that is prepared usingthe above production method and can electrochemically store and releasea large amount of lithium ions and a high-output-density,high-energy-density energy storage device that includes the electrodestructure and whose capacity decreases to a lesser extent even if theenergy storage device is repeatedly charged and discharged.

The expression “energy storage device” includes capacitors, rechargeablebatteries, a combination of a capacitor and a rechargeable battery, andthose devices having a power generation function.

Solution to Problem

The present inventor found the following: to maintain high capacity andachieve high charge-discharge efficiency and long charge-discharge cyclelife in an energy storage device, such as a lithium-ion rechargeablebattery, using an anode including particles containing silicon as a maincomponent (hereafter referred to as silicon anode), all the componentsof the silicon anode, that is, the particles themselves containingsilicon as a main component, a conductive additive, a binder and poresthat absorb the expansion of the volume during lithiation (lithiuminsertion), a current collector capable of enduring stress that occurswhen the volume is expanded, and an electrolyte solution additive forforming a stable SEI layer are important; and it is necessary toconsider a comprehensive measure such that the performance of thesilicon anode can be delivered. Further, the present inventor found thefollowing (1) to (5) and then found that a silicon anode (siliconelectrode) and lithium-ion rechargeable battery each having highcapacity and long-life could be realized.

(1) The inventor found that by providing a lithium composite oxide layercontaining metal elements selected from Al, Zr, Li, Mg, Ca, and La andthe Li element, which forms a more stable metal oxide than Si, on theinterface between the particles containing silicon as a main component(hereafter referred to as silicon particles) and the electrolytesolution, that is, on the silicon particles, it is possible to suppressthe formation of silicon oxide, to suppress the deposition of the metallithium on the silicon particles and the formation of inert LiF, Li₂O,Li₂CO₃, or the like due to the reaction between the electrolyte solutionand the lithium during a charge, and to produce an electrode having longcharge-discharge cycle life. It is conceivable that by coating thesilicon particles with the lithium composite metal oxide, which isthermodynamically more stable than silicon oxide, the oxidation of thesilicon particles is suppressed, lithium is deposited on the solid-solidinterface between the lithium composite oxide and silicon particles dueto the lithium-ion conductivity of the lithium composite oxide coatingduring a charge, and the reaction between the electrolyte solutioncomponents and lithium is suppressed. A specific method includesimmersing silicon particles in an alcohol solution of a nitrate ororganic acid salt of metals selected from Al, Zr, Mg, Ca, and La, or analkoxide, drying the silicon particles, and then firing the siliconparticles at 200 to 1000° C. to form a lithium composite metal oxidecoating on the silicon particles. A preferred method for furthersuppressing the formation of silicon oxide during milling of siliconincludes wet-milling silicon using alumina or zirconia beads in thealcohol solution to form silicon nanoparticles and, at the same time,form a lithium composite oxide coating thereon. Another preferred methodincludes mixing Li₂O or LiOH and an oxide or hydroxide of metalsselected from Al, Zr, Mg, Ca, and La with silicon particles, dry-millingthe silicon particles using a device, such as a planetary ball mill,vibrating mill, or attritor, and then firing the milled particles at 200to 1000° C. to form a lithium composite oxide coating on the siliconnanoparticles.

(2) Expanded graphite is flaked off by wet-milling or dry-milling it andthen used as a conductive additive; or the flaked graphite is mixed withsilicon particles and dried to form composite powder of a graphite piecehaving a high ratio of the surface-direction size to thethickness-direction size flaked off from the expanded graphite(hereafter referred to as “flake graphite”) and the silicon particles.Then, the composite powder was mixed with a binder to form an electrodelayer on the current collector and then the electrode layer was pressed.Thus, the inventor found that it is possible to dispose the flakegraphite in parallel with the current collector surface and to form along-life electrode having high conductivity and capable of maintainingpores formed due to the expansion of the volume and suppressing stressduring lithiation. A preferred method for producing a graphite piece(flake graphite) having a high surface-direction to thickness-directionsize ratio includes immersing expanded graphite in a solution of a saltselected from ammonium carbonate, ammonium hydrogencarbonate, andtetraalkylammonium hydroxide and then adding an organic acid to flakeoff the expanded graphite. The inventor found that the expanded graphitecan be easily flaked off due to the expansion of gas that occurs whenammonium carbonate, ammonium hydrogencarbonate, or tetraalkylammoniumhydroxide intercalated between the layers of the expanded graphitereacts with the organic acid. The reaction raw materials are gasified bytreating them at high temperature and therefore do not remain in theflaked-off expanded graphite.

(3) The inventor found that by complexing an electrode layer includingat least silicon particles, a conductive additive, and a binder bymixing one or more fibers selected from cellulose nanofiber, chitinnanofiber, chitosan nanofiber, and milled carbon fiber serving asreinforcing materials into the binder, it is possible to obtainmechanical strength with which the electrode can endure stress thatoccurs when the volume is expanded or contracted during a lithiuminsertion/extraction reaction. The inventor also found: the binder ispreferably a water-soluble polymer; by using water containingnanometer-sized (100 to 300 nm) bubbles (nano-bubble water) as a solventfor kneading the binder, silicon particles, conductive additive, andfiber, it is possible to form a porous electrode layer, to facilitatethe penetration of the electrolyte solution, to increase ionconductivity, and to suppress the expansion of the electrode layerduring lithiation.

(4) The inventor found that the charge-discharge cycle life can beextended by using, as the current collector of an electrode includingsilicon particles, a metal foil including a surface layer containingmilled carbon fiber and a binder, or a metal foil having a cross-sectionin the form of a wave which is selected from a sine wave, a triangularwave, a rectangular wave, a trapezoidal wave, and a saw-tooth wave andin which peaks and valleys are formed in line.

(5) The inventor found that the suppression of an increase in theinternal resistance of the battery associated with the charge-dischargecycle and the extension of the charge-discharge cycle life are achievedby using, as the electrolyte solution of a lithium-ion battery using anelectrode including silicon particles as an anode, an electrolytesolution containing, as additives, at least at least one or more vinylmonomers selected from the group consisting of styrene, 1-vinylnaphthalene, 2-vinyl naphthalene, acrylonitrile, N-vinyl pyrrolidone,and 4-vinyl pyrrolidone, 2-(2-propynyloxy)-3-vinylnaphthalene and atleast one or more divinyl monomers selected from the group consisting ofN,N′-methylene bisacrylamide, ethylene glycol dimethacrylate,1,2-divinylbenzene, and 1,3-divinylbenzene, 1,4-divinylbenzene, and/orone or more salts selected from an aluminum salt and a magnesium salt.The inventor also found that with respect to a monomer having anaromatic ring, of the monomers, a polymer that conducts lithium ions ina reduction reaction is formed on the active material surface; withrespect to a divinyl monomer, a stable crosslinking polymer is formed;and if the aluminum salt or magnesium salt is added, a layer of analuminum- or magnesium-containing oxide or fluoride is formed on theactive material. The inventor also found that the charge-discharge cyclelife of the energy storage device is extended by using the electrolytesolution containing those additives.

The inventor also found that the suppression of the decomposition of theorganic solvent in the electrolyte solution in a side reaction during acharge/discharge, a reduction in flammability, and an improvement insafety are achieved by dispersing lithium-ion solid electrolytenanoparticles in an electrolyte solution.

A first aspect of the present invention for solving the above problemsprovides an anode active material (anode material) for energy storagedevices capable of storing and releasing lithium ions. The anode activematerial includes secondary particles that are aggregates of 10 to 300nm primary particles containing silicon as a main component. The primaryparticles each include, as a surface layer, a composite metal oxidelayer containing at least one or more metal elements selected from atleast Al, Zr, Mg, Ca, and La and Li.

A second aspect of the present invention for solving the above problemsprovides a method for producing an anode active material for energystorage devices capable of electrochemically storing and releasinglithium ions. The method includes a step of immersing silicon particlesin an alcohol solution obtained by dissolving a nitrate or organic acidsalt of one or more metal elements selected from at least Al, Zr, Mg,Ca, and La and Li and then heat-treating the silicon particles. Apreferred method includes milling silicon in the alcohol solution of thenitrate or organic acid salt using a bead mill, drying the milledsilicon, and then heat-treating the resulting silicon to give an anodeactive material. One or more compounds selected from ethylene glycol,propylene glycol, polyethylene glycol, polypropylene glycol, andpolyvinyl pyrrolidone may be added to the alcohol solution. Preferably,the production method includes a step of mixing an oxide or hydroxide ofone or more metal elements selected from at least Al, Zr, Mg, Ca, and Laand lithium hydroxide or lithium oxide with silicon and dry-milling themixture using a media mill.

A third aspect of the present invention for solving the above problemsprovides an electrode structure for energy storage devices capable ofinserting and extracting lithium ions. The electrode structure includesthe active material containing silicon as a main component of thepresent invention, a conductive additive, a binder, and a currentcollector. The conductive additive is flake graphite which is obtainedby milling or flaking off expanded graphite and in which multiplegraphene sheets are stacked, and a plane in which six carbon atoms ofthe flake graphite have a regular hexagonal structure is oriented inparallel with a wide surface of the current collector.

The binder is a water-soluble polymer and is a composite of one or morefibers selected from a cellulose nanofiber, a chitin nanofiber, achitosan nanofiber, and milled carbon fiber. The current collector ofthe electrode structure of the present invention is a metal foil havinga cross-section in the shape of a wave which is selected from a sinewave, a triangular wave, a rectangular wave, a trapezoidal wave, and asaw-tooth wave and in which peaks and valleys are formed in line, or ametal foil having a layer made of milled carbon fiber and a binderformed thereon.

A fourth aspect of the present invention for solving the above problemsprovides an energy storage device that includes at least an anode, alithium-ion conductor, and a cathode formed of a lithium-transitionmetal compound and that can store and release lithium ions. The energystorage device uses the electrode structure of the present invention asan anode. The lithium-ion conductor is obtained by dispersing inorganicsolid lithium-ion conductor particles in an electrolyte solutionobtained by dissolving a lithium salt in an organic solvent. Thelithium-ion conductor may be obtained by adding at least an aluminumsalt or a magnesium salt to an electrolyte solution obtained bydissolving a lithium salt in an organic solvent. Preferably, thelithium-ion conductor is obtained by adding, to an electrolyte solutionobtained by dissolving a lithium salt in an organic solvent, at leastone or more vinyl monomers selected from the group consisting ofstyrene, 1-vinyl naphthalene, 2-vinyl naphthalene, acrylonitrile,N-vinyl pyrrolidone, 4-vinyl pyrrolidone, and2-(2-Propynyloxy)-3-vinylnaphthalene and at least one or more divinylmonomers selected from the group consisting of N,N′-methylenebisacrylamide, ethylene glycol dimethacrylate, 1,2-divinylbenzene,1,3-divinylbenzene, and 1,4-divinylbenzene.

The cathode of the energy storage device of the present inventionincludes lithium-transition metal compound particles coated with acomposite oxide containing one or more metal elements selected from atleast Al, Zr, Mg, Ca, and La and Li. Preferably, the flake graphite isused as a conductive additive included in the cathode.

Advantageous Effects of Invention

The anode active material capable of storing and releasing lithium ionsof the present invention suppresses the formation of an SEI layer duringlithiation, has high initial lithium insertion/extraction efficiency,and can store and release a large amount of lithium ions.

The anode active material (anode material) of the present invention andthe electrode structure including the conductive additive and binderreinforcing material of the present invention can endure the repetitionof the expansion/contraction associated with the electrochemicalinsertion/extraction of lithium and extend the life. The energy storagedevice using the electrochemical oxidation-reduction reaction of lithiumions of the present invention can achieve high initial charge/dischargeefficiency, high output density, and high energy density and can extendthe charge-discharge cycle life. Also, an energy storage device that canbe charged quickly can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A and 1B are schematic cross-sectional configuration diagram ofanode active material particles for storage power devices containingsilicon as a main component of the present invention.

FIG. 2 is a schematic cross-sectional configuration diagram of anelectrode structure of the present invention.

FIG. 3A, 3B and 3C are schematic diagram showing preferred shapes of acurrent collector included in the electrode structure of the presentinvention.

FIG. 4 is a schematic cross-sectional configuration diagram of a powerstorage device of the present invention.

FIG. 5 is a schematic cross-sectional configuration diagram of acoin-type cell, which is an example of the power storage device of thepresent invention.

FIG. 6 is a schematic cross-sectional configuration diagram of alaminated cell (pouch cell), which is an example of the power storagedevice of the present invention.

FIG. 7 is a schematic cross-sectional configuration diagram of acylindrical cell, which is an example of the power storage device of thepresent invention.

DETAILED DESCRIPTION

Now, the present invention will be described in detail.

An anode active material (anode material) for energy storage devicescapable of electrochemically storing and releasing lithium ions of thepresent invention includes secondary particles that are the aggregatesof 10-300 nm primary particles containing silicon as a main component,and the primary particles each include, as a surface layer, a compositemetal oxide layer containing at least one or more metal elementsselected from at least Al, Zr, Mg, Ca, and La and Li. In order toachieve a high-energy-density rechargeable battery, the silicon contentof the anode active material containing silicon as a main component ispreferably in a range of 50% to 99% by weight. The production method ofthe anode active material includes a step of immersing silicon particlesin an alcohol solution obtained by dissolving a nitrate or organic acidsalt of one or more metal elements selected from at least Al, Zr, Mg,Ca, and La and Li and then heat-treating the silicon particles. Theproduction method may include a step of bead-milling silicon in analcohol solution of the nitrate or organic acid salt or an alkoxide,drying the milled silicon, and then heat-treating the dried silicon togive an anode active material. One or more compounds selected fromethylene glycol, propylene glycol, polyethylene glycol, polypropyleneglycol, and polyvinyl pyrrolidone may be added to the alcohol solution.These compounds have the effect of facilitating the formation of acomposite oxide by coordinating with the metal elements. Preferably, theproduction method includes a step of mixing an oxide or hydroxide of oneor more metal materials selected from at least Al, Zr, Mg, Ca, and La,lithium oxide or lithium hydroxide, and silicon powder and dry-millingthe mixture using a media mill. Another preferred method includesintroducing silicon powder serving as a raw material, powder of one ormore metals selected from Al, Zr, Mg, Ca, and La or a compound thereof,and lithium compound powder into thermal plasma to form a composite ofsilicon nanoparticles and lithium composite metal oxide. Preferably, thecompound of the metals or the lithium compound is an oxide, hydroxide,organic acid salt, or organic metal.

An electrode structure for energy storage devices capable of insertingand extracting lithium ions of the present invention includes the activematerial containing silicon as a main component of the presentinvention, a conductive additive, a binder, and a current collector. Theconductive additive is flake graphite which is obtained by milling orflaking off expanded graphite and in which multiple graphene sheets arestacked, and a plane in which six carbon atoms of the flake graphitehave a regular hexagonal structure is oriented in parallel with a widesurface of the current collector. The production method of the flakegraphite includes a step of immersing expanded graphite in a solution ofa salt selected from ammonium carbonate, ammonium hydrogencarbonate, andtetraalkylammonium hydroxide and then adding an organic acid to flakeoff the expanded graphite. Preferably, the binder is a water-solublepolymer and is a composite of one or more fibers selected from acellulose nanofiber, a chitin nanofiber, a chitosan nanofiber, andmilled carbon fiber. To form many pores in the electrode layer (mainly,the binder) of the electrode structure, water containing nanometer-sized(100 to 300 nm) bubbles is preferably used as a solvent for kneading theactive material containing silicon as a main component, conductiveadditive, and binder. The current collector of the electrode structureof the present invention is a metal foil having a cross-section in theshape of a wave which is selected from a sine wave, a triangular wave, arectangular wave, a trapezoidal wave, and a saw-tooth wave and in whichpeaks and valleys are formed in line, or a metal foil having a layermade of milled carbon fiber and a binder formed thereon.

An energy storage device of the present invention is an energy storagedevice that includes at least an anode, a lithium-ion conductor, and acathode formed of a lithium-transition metal compound and that canintercalate and deintercalate lithium ions. This energy storage deviceuses the electrode structure of the present invention as an anode. Thelithium-ion conductor is obtained by dispersing inorganic solidlithium-ion conductor particles in an electrolyte solution obtained bydissolving a lithium salt in an organic solvent. The lithium-ionconductor may be obtained by adding at least one or more salts selectedfrom an aluminum salt and a magnesium salt in an electrolyte solutionobtained by dissolving a lithium salt in an organic solvent. Preferably,the lithium-ion conductor is obtained by adding, to an electrolytesolution obtained by dissolving a lithium salt in an organic solvent, atleast one or more vinyl monomers selected from the group consisting ofstyrene, 1-vinyl naphthalene, 2-vinyl naphthalene, acrylonitrile, andN-vinyl pyrrolidone, 4-vinyl pyrrolidone, and2-(2-Propynyloxy)-3-vinylnaphthalene and at least one or more divinylmonomers selected from the group consisting of N,N′-methylenebisacrylamide, ethylene glycol dimethacrylate, 1,2-divinylbenzene,1,3-divinylbenzene, and 1,4-divinylbenzene. Preferably, the cathode ofthe energy storage device of the present invention includeslithium-transition metal compound particles coated with a compositemetal oxide containing one or more metal elements selected from at leastAl, Zr, Mg, Ca, and La and Li.

Now, embodiments of the present invention will be described withreference to FIGS. 1 to 7.

[Anode Active Material Containing Silicon as Main Component]

An embodiment of the anode active material for power storage devices ofthe present invention that includes a composite oxide layer formed of atleast one or more metal elements selected from at least Al, Zr, Mg, Ca,and La and Li and that contains silicon as a main component will bedescribed more specifically with reference to FIG. 1A and 1B.

FIG. 1A is a cross-sectional schematic view of an active materialparticle containing silicon as a main component, and FIG. 1B is across-sectional schematic view of a secondary particle that is theaggregate of multiple active material particles containing silicon as amain component. In FIG. 1A and 1B, reference signs 101 and 103 representactive material particles containing silicon as a main component, andreference signs 102 and 104 represent composite metal oxide layerscontaining at least one or more metal elements selected from at leastAl, Zr, Mg, Ca, and La and Li. Ideally, the entire surface of the activematerial particle containing silicon as a main component is coated withsuch a composite metal oxide layer, but the entire surface need notnecessarily be coated. The material of the active material particlecontaining silicon as a main component is silicon or silicon alloy.Preferably, the silicon is metal (grade) silicon having a purity on theorder of 93 to 99.9% obtained by reducing silicon dioxide, or solarcell-grade silicon or semiconductor-grade silicon having a higherpurity. More preferably, the silicon is metal silicon since metalsilicon is cheap and is easily milled due to its small crystal particlesize. Metal silicon contains 0.5% by weight or less of iron, 0.5% byweight or less of aluminum, and 0.3% by weight or less of calcium.Preferably, the silicon alloy is an alloy of silicon and at least atransition metal. The diameter of the active material particlecontaining silicon as a main component and serving as a primary particleis preferably 10 to 300 nm, more preferably 10 to 100 nm. The thicknessof the composite metal oxide layer is preferably in a range of 2 to 100nm, more preferably in a range of 2 to 20 nm. Although not shown in FIG.1A and 1B, the anode active material particles of the present inventionmay be entirely or partially coated with an amorphous carbon layer.Also, the composite metal oxide layer may be a composite with amorphouscarbon. Preferably, the active material particles have smallerdiameters, since the substantial current density on the active materialparticle surface is reduced during the charge/discharge of the battery;lithium makes oxidation-reduction reactions uniformly; and the volumeexpands or contracts more uniformly during the insertion/extraction oflithium ions. On the other hand, it is preferred to select a particlesize range such that less silicon oxide is formed, since silicon oxideis easily formed in the step for reducing the particle size.

A specific production method of the anode active material of the presentinvention includes dispersing fine powder consisting of particlescontaining silicon as a main component in an alcohol solution obtainedby dissolving a nitrate or organic acid salt of one or more metalelements selected from at least Al, Zr, Mg, Ca, and La and Li, dryingthe particles, and then heat-treating the particles in an atmosphere ofan inert gas preferably at 200 to 1000° C., more preferably at 300 to900° C. The production method may include a step of wet-milling powderconsisting of particles containing silicon as a main component using abead mill while using, as a medium, an alcohol solution obtained bydissolving a nitrate or organic acid salt of one or more metal elementsselected from at least Al, Zr, Mg, Ca, and La and Li, drying theparticles, and then heat-treating the particles in an atmosphere of aninert gas preferably at 200 to 1000° C., more preferably at 300 to 900°C. Preferably, one or more compounds selected from ethylene glycol,propylene glycol, polyethylene glycol, polypropylene glycol, andpolyvinyl pyrrolidone is added to the alcohol solution. These compoundsfacilitate the formation of a composite metal oxide by coordinating withmetal ions. Also, part of the compounds is thermally decomposed and thusforms a composite layer with the composite metal oxide as amorphouscarbon, remains on the particles containing silicon as a main component,and contributes to an improvement in the conduction of electrons.

[Electrode Structure]

The electrode structure of the present invention is an electrodestructure for energy storage devices capable of inserting and extractinglithium ions and includes the active material containing silicon as amain component of the present invention, a conductive additive, abinder, and a current collector. An embodiment of the electrodestructure of the present invention will be described more specificallywith reference to FIG. 2.

FIG. 2 is a cross-sectional schematic view of the electrode structure ofthe present invention. In FIG. 2, reference sign 200 represents acurrent collector; reference sign 201 represents an active materialcontaining silicon as a main component; reference sign 202 representsflake graphite as a conductive additive; reference sign 203 represents abinder; reference sign 204 represents a fiber-shaped reinforcingmaterial; reference sign 205 represents a electrode layer; and referencesign 206 represents an electrode structure. Preferably, the conductiveadditive is flake graphite which is obtained by milling or flaking offexpanded graphite and in which multiple graphene sheets are stacked, anda plane in which six carbon atoms of the flake graphite have a regularhexagonal structure is oriented in parallel with a wide surface of thecurrent collector. Carbon black, carbon nanofiber, carbon nanotube, orthe like may be further added as a conductive additive. A preferredproduction method of the flake graphite serving as a conductive additiveused in the electrode structure of the present invention includesimmersing expanded graphite in a solution of a salt selected fromammonium carbonate, ammonium hydrogencarbonate, and tetraalkylammoniumhydroxide and then adding an organic acid to flake off the expandedgraphite. As another production method, expanded graphite may be flakedoff using a mill that includes a bead mill or ball mill and uses acavitation effect.

Preferably, the binder 203 is a water-soluble polymer and is mixed withone or more fiber-shaped reinforcing materials 204 selected from acellulose nanofiber, a chitin nanofiber, a chitosan nanofiber, andmilled carbon fiber. The cellulose nanofiber, chitin nanofiber, andchitosan nanofiber are fibers having a diameter of about 4 to 100 nm anda length of 2 μm or more and can be prepared using the aqueous countercollision method or the like. Milled carbon fiber is obtained by millinga carbon fiber thread into short fibers. Preferred milled carbon fiberhas a diameter of the order of 7 μm and a length of the order of 30 μm,but not limited thereto. By mixing the fiber-shaped material with thebinder, it is possible to improve the mechanical strength, as well as toimprove the strength against stress associated with the expansion orcontraction of the volume which occurs when the active materialcontaining silicon as a main component stores or releases lithium. Ifmilled carbon fiber is mixed with the binder, it is possible to improvethe mechanical strength of the electrode structure, as well as theconductivity thereof. The reason is that milled carbon fiber is lesslikely to be agglomerated and more likely to be dispersed and has highconductivity compared to a carbon nanofiber or carbon nanotube.

Typical examples of a water-soluble polymer used as the binder includesodium alginate, sodium carboxymethylcellulose, carboxymethylcellulose,sodium polyacrylate, polyacrylic acid, polyvinyl alcohol, chitin, andchitosan. Preferably, the binder is crosslinked in order to increase themechanical strength. Note that if polyvinyl alcohol, which has highwater absorbing properties, is used as a binder for the electrode of anenergy storage device as it is, a hydrogen gas occurs during a charge,thereby increasing the pressure in the housing of the energy storagedevice. To suppress this phenomenon, it is preferred to crosslinkpolyvinyl alcohol using a crosslinking agent to reduce hydroxyl groups.Polyacrylic acid and polyvinyl alcohol may be mixed so that thesecomponents are crosslinked by a hydrogen bond.

Preferably, the current collector used in the electrode structure of thepresent invention is a metal foil having a cross-section in the shape ofa wave which is selected from a sine wave, a triangular wave, arectangular wave, a trapezoidal wave, and a saw-tooth wave and in whichpeaks and valleys are formed in line, or a metal foil having a layermade of milled carbon fiber and a binder formed thereon. FIG. 3A is aschematic diagram showing an example of a current collector having across-sectional shape of a triangular wave; FIG. 3B is a schematicdiagram showing an example of a current collector having across-sectional shape of a rectangular wave; and FIG. 3C is a schematicdiagram showing an example of a current collector having thereon a layerformed of milled carbon fiber and a binder. Reference signs 301, 302,and 303 represent metal foils; reference sign 304 represents a binder;reference sign 305 represents milled carbon fiber; and reference sign306 represents a layer formed of milled carbon and a binder. The metalfoil must be formed of a material that is not melted but rather isstable when the energy storage device makes a charge/discharge reaction.Specific examples of the material include copper, stainless steel,titan, and nickel. The above cross-section shape can be easily formed bypressing a metal foil through a roll press having protrusions andrecesses patterned thereon. Examples of the material of the binder usedin the layer formed of milled carbon fiber and binder include sodiumalginate, sodium carboxymethylcellulose, carboxymethylcellulose, sodiumpolyacrylate, polyacrylic acid, polyvinyl alcohol, chitin, chitosan,polyimide, polyamideimide, polyurethane, and epoxy resin. Preferably,the milled carbon fiber has a diameter on the order of 7 μm and a lengthon the order of 30 to 150 μm.

If a metal foil having a cross-sectional shape of a wave as describedabove is used, the current collector of the electrode structure of thepresent invention can follow the expansion or contraction of the volumecaused by the lithiation or delithiation by the anode active material ofthe present invention containing silicon as a main component when theenergy storage device makes a charge/discharge reaction and can reducestress that occurs due to the expansion or contraction of the volume.Also, if a metal foil having a layer formed of milled carbon fiber and abinder formed thereon are used as the current collector of the electrodestructure of the present invention, it is possible to maintain themechanical strength that can withstand stress that occurs due to theexpansion or contraction of the volume when the energy storage device ischarged or discharged.

Method for Producing Electrode Structure

A method for producing the electrode structure of the present inventionincludes the following steps.

First, the active material 201 coated with the composite metal oxide andcontaining silicon as a main component of the present invention, theflake graphite 202 serving as a conductive additive, the binder 203, andbio-nanofibers (cellulose nanofiber, chitin nanofiber, chitosannanofiber) serving as fiber-shaped reinforcing materials 204 and/ormilled carbon fiber are mixed at a predetermined mixing ratio; a solventfor the binder is added; and the mixture is kneaded using a kneader toprepare a slurry. Then, the resulting slurry is applied to the currentcollector 200 using a coater, dried, and then heat-treated under areduced pressure or in an atmosphere of an inert gas to give anelectrode structure. The heat treatment temperature is preferably in arange of 100 to 300° C. By using a water-soluble polymer as the binderand water containing nanometer-sizes (100 to 300 nm) bubbles (nanobubble water) as a solvent for the binder, a porous electrode layer canbe formed. The gas in the bubbles is preferably a nitrogen gas. If theporous electrode structure of the present invention is used as the anodeof an energy storage device that uses reduction/oxidation reactions oflithium ions, it is possible to reduce the expansion of the volume ofthe active material caused by lithiation during a charge, to facilitatethe penetration of the electrolyte solution, and to reduce the internalimpedance of the energy storage device.

Preferably, the solid contents of the active material coated with thecomposite metal oxide and containing silicon as a main component of thepresent invention, the flake graphite serving as a conductive additive,the binder, and the bio nanofiber and/or milled carbon fiber are 20 to80% by weight, 10 to 50% by weight, 3 to 20% by weight, and 1 to 5% byweight, respectively, with respect to the entire solid content of theslurry. Preferably, the electrode layer 205 has a thickness of 10 to 50μm and a density of 0.8 to 2.0 g/cm³.

[Energy Storage Device]

The energy storage device of the present invention is an energy storagedevice that uses reduction/oxidation reactions of lithium ions andincludes at least the electrode structure of the present inventionserving as an anode, an ion conductor, and a cathode formed of alithium-transition metal compound. An embodiment of the electrodestructure of the present invention will be described more specificallywith reference to FIG. 4.

In FIG. 4, reference sign 401 represents an anode current collector,reference sign 402 represents an anode active material layer, referencesign 403 represents an anode, reference sign 404 represents alithium-ion conductor, reference sign 405 represents a cathode currentcollector, reference sign 406 represents a cathode active materiallayer, reference sign 407 represents a cathode, reference sign 408represents a housing, reference sign 409 represents an anode lead,reference sign 410 represents an anode terminal, reference sign 411represents a cathode lead, and reference sign 412 represents a cathodeterminal.

The anode 403 is the electrode structure of the present invention. Thelithium-ion conductor 404 is preferably obtained by dispersing inorganicsolid lithium-ion conductor particles in an electrolyte solutionobtained by dissolving a lithium salt in an organic solvent. Examples ofthe inorganic solid lithium-ion conductor include a sulfur-basedamorphous electrolyte represented by Li₂S—P₂O₅, sulfur-containing glass,lithium nitride Li₃N, a material having a NASICON crystal structurerepresented by Li_(1+x+y)Al_(x)Ti_(2−x)Si_(y)P_(3−y)O₁₂ (x=0.3, y=0.2),a material having a garnet structure represented by Li₇La₃Zr₂O₁₂, and agermanium-phosphorus-sulfur compound represented by Li₁₀GeP₂S₁₂.Preferably, the average diameter of the primary particles of theinorganic solid electrolyte is in a range of 10 to 300 nm. A slurryelectrolyte solution obtained by dispersing inorganic solid lithium-ionconductor particles can reduce the organic solvent content, suppress adecomposition reaction acting as a side reaction during a charge,prevent an increase in the SEI layer, suppress an increase in electroderesistance, extend the charge-discharge cycle life, and improve safety.Preferably, an electrolyte solution of the lithium-ion conductor 404,which is obtained by dissolving a lithium salt in an organic solvent,contains at least one or more salts selected from an aluminum salt and amagnesium salt as additives. If an aluminum salt or magnesium salt isadded, it is possible to easily form an aluminum- ormagnesium-containing oxide coating or fluoride coating on the anodeduring a cathodic reaction, to increase the resistance of a portionwhich is more likely to suffer an internal short-circuit and thus toprevent an internal short-circuit, as well as to suppress an electrolytereaction of the organic solvent contained in the electrolyte solution,to prevent an increase in the SEI layer, to suppress an increase inelectrode resistance, and to extend the charge-discharge cycle life.Preferably, the amount of an aluminum salt or magnesium salt added tothe electrolyte solution is 0.1 to 3% by weight. Preferably, at leastone or more vinyl monomers selected from the group consisting ofstyrene, 1-vinyl naphthalene, 2-vinyl naphthalene, acrylonitrile,N-vinyl pyrrolidone, 4-vinyl pyrrolidone, and2-(2-Propynyloxy)-3-vinylnaphthalene and at least one or more divinylmonomers selected from the group consisting of N,N′-methylenebisacrylamide, ethylene glycol dimethacrylate, 1,2-divinylbenzene,1,3-divinylbenzene, and 1,4-divinylbenzene are added to the electrolytesolution obtained by dissolving a lithium salt in an organic solvent.The vinyl monomers can form a polymer that stably conducts lithium ionsonto the anode during a charge reaction, prevent an increase in thethickness of the SEI layer, suppress an increase in electroderesistance, and extend the charge-discharge cycle life.

Preferably, the cathode active material layer 406 of the cathode 407includes lithium-transition metal compound particles coated with acomposite metal oxide containing one or more metal elements selectedfrom at least Al, Zr, Mg, Ca, and La and Li. The composite metal oxidelayer containing at least one or more metal elements selected from atleast Al, Zr, Mg, Ca, and La and Li can prevent the dissolution of thetransition metal element from the lithium-transition metal compoundserving as a cathode active material in the electrolyte solution, whichacts as a side reaction, during a charge/discharge. Further, thecomposite metal oxide layer can prevent an increase in the thickness ofthe SEI layer, suppress an increase in electrode resistance, and extendthe charge-discharge cycle life. It is also preferred to reduce theelectrical resistance of the cathode by using the above flake graphiteas a conductive additive for the cathode.

[Method for Producing Energy Storage Device]

A method for producing the energy storage device will be described withreference to FIG. 4. An energy storage device is assembled as follows:first, the lithium-ion conductor 404 is sandwiched between the anode 403including the anode current collector 401 and anode active materiallayer 402 and the cathode 407 including the cathode current collector405 and cathode active material layer 406 to form a multilayer electrodegroup; the electrode group is inserted into a battery jar (housing,package) 408 in an atmosphere of dried air or dried inert gas whosedew-point temperature is managed sufficiently; then the electrodes andelectrode terminals 410 and 412 are connected through the electrodeleads 409 and 411, respectively; and the battery jar 408 is sealed. Asused herein, the term “energy storage device” covers capacitors,rechargeable batteries, combinations of a capacitor and a rechargeablebattery, or those having a power generation function.

Examples of the specific cell shape of the energy storage deviceproduced in the present invention include flat, cylindrical, rectangularparallelepiped, and sheet shapes. Examples of the cell structure includemonolayer type, multilayer type, and spiral type. Among these, a spiral,cylindrical cell, which is obtained by winding the electrode in multiplelayers with a separator between the anode and cathode, has a largerelectrode area and can pass a large current during a charge/discharge.With respect to rectangular parallelepiped- and sheet-shaped cells, thestorage space of a device which store a cell, can be effectively used.

FIG. 5 shows a schematic cross-sectional view of a monolayer flat(coin-type) cell; FIG. 6 shows a schematic cross-sectional view of alaminated cell (pouch cell); and FIG. 7 shows a schematiccross-sectional view of a spiral cylindrical cell. Each cell includes ananode including an anode current collector and an anode active materiallayer, a cathode including a cathode current collector and a cathodeactive material layer, and an ion conductor between the anode andcathode. In FIGS. 5, 6, and 7, reference signs 501, 601, and 701represent anode current collectors; reference signs 502, 602, and 702represent anode active material layers; reference signs 503, 603, and703 represent ion conductors; reference signs 504, 604, and 704represent cathode current collectors; reference signs 505, 605, and 705represent cathode active material layers; reference sign 708 representsan anode lead; reference sign 709 represents a cathode lead; referencesigns 506 and 606 represent anode terminals; reference signs 507, 607,and 707 represent cathode terminals; reference sign 608 represents abattery jar (housing, package); reference signs 508 and 713 representgaskets; reference sign 710 represents a safety valve; reference signs711 and 712 represent insulating plates; and reference sign 706represents a battery jar serving as an anode terminal.

[Cathode]

These cathodes have structures similar to the electrode structure shownin FIG. 2. In each cathode, a cathode active material layer including alithium-transition metal compound serving as an active material, abinder, and a conductive additive such as carbon black is formed on thecathode current collector.

Examples of a cathode active material for rechargeable batteries usingthe oxidation and reduction of lithium ions include transition metaloxides, transition metal phosphate compounds, lithium-transition metaloxides, and lithium-transition metal phosphate compounds. Preferably,the cathode active material contains a transition metal element, such asNi, Co, Mn, Fe, Cr, V, or the like, as a main element. The cathodeactive material may be mixed with an oxide or composite oxide containingan element selected from Mo, W, Nb, Ta, V, B, Ti, Ce, Al, Ba, Zr, Sr,Th, Mg, Be, La, Ca, and Y as a main component. Preferably, the cathodeactive material particles included in the cathode are lithium-transitionmetal compound particles coated with a composite oxide containing one ormore metal elements selected from at least Al, Zr, Mg, Ca, and La andLi. Preferably, conductive additives included in the cathode include atleast flake graphite which is obtained by milling or flaking offexpanded graphite and in which graphene sheets are stacked in multiplelayers.

High-specific-surface-area and/or porous carbon is used as a cathodeactive material for capacitors. Examples of thehigh-specific-surface-area and/or porous carbon include a carbonmaterial obtained by carbonizing an organic polymer in an atmosphere ofan inert gas and a carbon material having pores formed by treating thiscarbonized material with an alkali or the like. Also, mesoporous carbonobtained by inserting an organic polymer material into a template, suchas an oxide, produced in the presence of an amphiphilic surfactant andhaving oriented pores, carbonizing the material, and eliminating a metaloxide by etching may be used as a cathode active material. Preferably,the specific surface area of the carbon material is 10 to 3000 m²/g.Examples of an conductive additive other than flake graphite include acarbon nanofiber (nanometer-order carbon fiber), a carbon nanotube,graphene, a high-specific-surface-area and/or porous carbon material,such as graphite, having a specific surface area increased by milling orthe like, carbon called carbon black, such as acetylene black or Ketjenblack, and high-specific surface-area metal oxides, such as manganeseoxides (including semimetal oxides).

Examples of the binder include fluororesins such as polyvinylidenefluoride, polyacrylate, polyamic acid (polyimide precursor), polyimide,polyamide-imide, epoxy resins, and styrene butadienecopolymer-carboxymethylcellulose.

Preferably, the material of the current collector is a material that hashigh electrical conductivity and is inert to a battery reaction.Preferred materials include materials containing one or more metalmaterials selected from aluminum, nickel, iron, stainless steel,titanium, and platinum. A more preferable material is aluminum, which ischeap and has low electrical resistance. The shape of the currentcollector is a sheet shape. The “sheet shape” here is not limited to apractical range with respect to the thickness and includes a form called“foil” having a thickness of about 5 to 100 μm. For example, a mesh,spongy, or fiber-shaped member, a punching metal, a metal having athree-dimensional uneven pattern formed on the front and back sidesthereof, or an expand metal may be used as long as it has a sheet shape.For example, a plate-shaped or foil-shaped metal having athree-dimensional uneven pattern formed thereon can be produced bytransferring a microarray pattern or line-and-space pattern formed on ametallic or ceramic roll to a plate-shaped or foil-shaped metal byapplying a pressure to the roll. In particular, an energy storage deviceusing a current collector having a three-dimensional uneven patternformed thereon has the following advantageous effects: a reduction inthe substantial current density with respect to the electrode areaduring a charge/discharge; an improvement in the adherence to theelectrode layer; an improvement in the mechanical strength; animprovement in the rate capability during a charge/discharge; and animprovement in the charge-discharge cycle life.

[Ion Conductor]

If the energy storage device of the present invention is a rechargeablebattery, examples of the ion conductor thereof include a separatorholding an electrolyte solution (an electrolyte solution prepared bydissolving an electrolyte in a solvent), a solid electrolyte, asolidified electrolyte obtained by gelling an electrolyte solution usinga polymer gel or the like, a composite of a polymer gel and a solidelectrolyte, and a lithium-ion conductor such as an ionic liquid. Inpractice, a separator for preventing electrical short-circuit is placedbetween the anode and cathode, and the micropores of the separator areimpregnated with the ion conductor.

The separator is a resin film having a micropore structure or nonwovenfabric structure. Preferably, the resin material is polyolefin such aspolyethylene or polypropylene, polyimide, polyamide-imide, or cellulose.To increase heat resistance, the microporous resin film may be coatedwith a metal oxide particle containing a layer that contains alumina,zirconia, titania, or the like and that passes lithium ions.

The conductivity of the ion conductor used in a rechargeable battery ispreferably 1×10⁻³ S/cm or more, more preferably 5×10⁻³ S/cm or more on a25° C. basis.

Examples of the electrolyte include salts containing lithium ions (Li⁺)and Lewis acid ions [BF₄ ⁻, PF₆ ⁻, AsF₆ ⁻, ClO₄ ⁻, CF₃SO₃ ⁻, BPh₄ ⁻ (Ph:phenyl group)], mixed salts thereof and ionic liquid.

Preferably, such a salt is sufficiently dehydrated and deoxidated, forexample, by heating it under a reduced pressure. An electrolyte preparedby dissolving the above lithium salt in an ionic liquid may be used.Examples of a solvent for the electrolyte include acetonitrile,benzonitrile, propylene carbonate, ethylene carbonate, dimethylcarbonate, diethyl carbonate, ethylmethyl carbonate, dimethylformamide,tetrahydrofuran, nitrobenzene, dichloroethane, di-ethoxyethane, 1,2-dimethoxy-ethane, chlorobenzene, γ-butyrolactone, dioxolane,sulfolane, nitromethane, dimethylsulphide, dimethyl sulfoxide,3-methyl-2-oxazolidinone, 2-methyl tetrahydrofuran, 3-propylsydnone,sulfur dioxide, and mixed solutions thereof. A solvent having astructure in which hydrogen of the above solvent is substituted byfluorine may be used. Ionic liquid may also be used.

Preferably, the solvent is dehydrated using, for example, activealumina, molecular sieve, phosphorus pentoxide, or calcium chloride.Depending on the solvent, it is preferred to impurity-eliminate anddehydrate the solvent by distilling it in the coexistence of an alkalimetal in an inert gas. To achieve high ion conductivity, the electrolyteconcentration of an electrolyte solution prepared by dissolving theelectrolyte in the solvent is preferably in a range of 0.5 to 3.0mol/liter.

To suppress the reaction between the electrode and electrolyte solution,it is preferred to add an organic fluorine compound for forming a stablefluoride on the electrode surface, such as fluoro-ethylene carbonate ordifluoro-ethylene carbonate.

Preferably, the solidified electrolyte is one obtained by gelling theelectrolyte solution using a gelling agent and then solidifying it.Preferably, the gelling agent is a liquid-absorptive, porous materialthat absorbs the solvent of the electrolyte and thus swells, such aspolymer or silica gel. Examples of the polymer include polyethyleneoxide, polyacrylonitrile, polymethyl methacrylate, vinylidenefluoride-hexafluoropropylene copolymer, and polyethylene glycol.Preferably, the polymer has a crosslinking structure.

[Gasket]

Examples of the material of the gaskets (508, 713) include afluororesin, a polyolefin resin, a polyamide resin, a polysulfone resin,and various types of rubber. Examples of the method for sealing thebattery include “swaging” using a gasket as shown in FIGS. 5 and 7, aswell as glass sealing, an adhesive, and welding.

Examples of the material of the insulating plates (711, 712) in FIG. 7include various types of organic resin materials and ceramic.

Battery Jar

Examples of the material of the battery jars (housing, package) (408,608, 706) include stainless steel, an aluminum alloy, a titanium cladstainless material, a copper clad stainless material, and a nickelplating sheet. An aluminum laminate film, which is a laminate of a resinfilm and an aluminum foil, and the like are also often used.

Other examples of the material of the battery jar include metals otherthan stainless steel, such as zinc, plastic such as polypropylene, and acomposite of a metal or glass fiber and plastic.

[Safety Valve]

A lithium rechargeable battery includes a safety valve (710) as a safetymeasure for eliminating the pressure when the internal pressure of thebattery is increased. The safety valve is, for example, a rapture foil,rubber, spring, or a metal ball.

EXAMPLES

Hereafter, the present invention will be described in more detail withreference to Examples.

[Preparation of Anode Active Material for Energy Storage DevicesContaining Silicon as Main Component]

Example M1

One part by weight of lithium citrate and 26.7 parts by weight ofaluminum nitrate (9 hydrate) with respect to 100 parts by weight ofsilicon were added to a solution obtained by dispersing 10% by weight ofmetal silicon powder having a particle size of 10 μm or less in ethanol.Further, a small amount of polyethylene glycol was added to give a mixedsolution. While circulating the resulting mixed solution, theraw-material silicon was milled using a first wet bead mill filled withzirconia beads having a particle size of 0.5 mm until the averageparticle size became 0.5 μm or less. Then, while circulating theresulting raw material, it was milled using a second wet bead millfilled with zirconia beads having a particle diameter of 0.03 mm untilthe average particle size became 100 nm or less, to give a milleddispersion slurry.

Then, the resulting milled dispersion slurry was spray-dried using aspray dryer in an atmosphere of a nitrogen gas at 150° C. to givepowder. Then, the powder was heat-treated in a firing furnace in anatmosphere of a nitrogen gas at 600° C. for 30 min to give alithium-aluminum composite oxide-coated silicon composite.

Example M2

One hundred parts by weight of metal silicon powder having a particlesize of 100 μm or less, 1.5 parts by weight of lithium hydroxide (1hydrate), and 9.1 parts by weight of aluminum oxide were mixed using ahigh-speed planetary ball mill, milled using a vibrating mill for 10 h,and then heat-treated in an atmosphere of nitrogen at 800° C. for 30 minto give a lithium-aluminum composite oxide-coated silicon composite.Typically, when dry-pulverized silicon is extracted in the air, it needsto be gradually oxidized, since red hot occurs due to oxidation. On theother hand, the silicon powder of Example M2 could be extracted safelywithout having to gradually oxidize it, since the silicon powder wascoated with the oxide.

Comparative Example M1

Silicon powder was obtained as in Example M1 except that lithiumcitrate, aluminum nitrate (9 hydrate), or ethylene glycol was not added.

Comparative Example M2

Silicon was pulverized as in Example 2 except that lithium hydroxide (1hydrate) or aluminum oxide was not mixed, and then gradually oxidized togive silicon powder.

Analysis of Particles Containing Silicon as Main Component

The resulting powder containing silicon as a main component was analyzedusing an X-ray photoelectron spectroscopy (XPS), a scanning electronmicroscope (SEM), a transmission electron microscope (TEM), and anenergy dispersive X-ray spectrometer (EDS) and electron energy-lossspectrometer (EELS) accompanying the transmission electron microscopeand then evaluated.

In the samples of Examples M1 and M2, the silicon particle surface layerwas coated with a composite oxide of lithium and aluminum, and anoxidized coating having a thickness of 2 to 10 nm was identified. InExample M1, carbon was also identified on the surface layer.

In Comparative Example M1, a silicon layer in which hydrocarbon andoxygen were combined was identified on the surface layer; in ComparativeExample M2, a silicon oxide layer was identified on the surface layer.The descending order of the amount of silicon oxide is as follows:Comparative Example M2>Comparative Example M1> Example M2>Example M1.

[Preparation of Flake Graphite]

Reference Example G1

A solution was prepared by dissolving ammonium hydrogencarbonate indeionized water, and expanded graphite having an average particle sizeof 15 μm was dispersed in the solution. With stirring, a solutionobtained by dissolving citric acid in deionized water was added to causethe ammonium hydrogencarbonate penetrating the expanded graphite and thecitric acid to react with each other to generate carbon dioxide. Theexpanded graphite was flaked off using the pressure of the generatedcarbon dioxide to prepare a dispersion of the flake graphite.

[Preparation of Current Collector]

Reference Example C1

An electrolytic copper foil having a thickness of 12 μm was passedbetween a chrome-plated metal roll having a pitch of 50 μm and a depthof 25 μm and subjected to V-shaped line groove treatment by electronicengraving and a rubber roll, thereby forming a current collector havinga cross-sectional shape of a triangular wave (actually, a shape in whichpeaks and valleys are roundish and which is similar to a sine wave).

Reference Example C2

Deionized water was added and to 95 parts by weight of milled carbonfiber having a diameter of 7 μm and a length of 130 μm and 5 parts byweight of sodium alginate, and kneaded to prepare a slurry. The slurrywas applied to both sides of a flat electrolytic copper foil having athickness of 12 μm and dried at 100° C. to form a current collector.

[Production of Electrode Structure]

Example N1

A binder solution was prepared by dissolving 10% by weight of sodiumalginate in an aqueous dispersion of 1% by weight of cellulosenanofiber. Then, the binder solution, 60 parts by weight of the powdercontaining silicon as a main component of Example M1, 27 parts by weightof the flake graphite prepared in Reference Example G1, and 2 parts byweight of acetylene black were mixed (so that the solid content ofsodium alginate became 10 parts by weight and the solid content of thecellulose nanofiber became 1 part by weight). Then, nano bubble water ofa nitrogen gas was added and kneaded to prepare a slurry for forming anelectrode layer. The resulting slurry was applied to both sides of thecopper foil of Reference Example C1 using a coater, then dried at 110°C. for 0.5 h, and further dried under a reduced pressure at 150° C.Then, the thickness and density were adjusted using a roll press to givean electrode structure in which an electrode active material layerhaving a thickness of 30 μm and a density of 1.2 g/cm³ was formed on acurrent collector formed of a copper foil. Then, the electrode structurewas cut into a predetermined size; then a nickel lead was welded to thetab of the copper foil as a current collector using a spot welder; and alead terminal was extracted to produce an electrode.

Example N2

An electrode was produced as in Example N1 except that the powder ofExample M2 was used in place of the powder of Example M1 containingsilicon as a main component.

Example N3

An electrode was produced as in Example N1 except that graphite powderhaving a particle size of 5 μm was used in place of the flake graphiteof Reference Example G1.

Example N4

An electrode was produced as in Example N1 except that a cellulosenanofiber was not mixed.

Example N5

An electrode was produced as in Example N1 except that nano bubble waterwas not added, but rather deionized water not including nano bubbles wasused.

Example N6

An electrode was produced as in Example N1 except that the currentcollector coated with milled carbon fiber of Reference Example C2 wasused in place of the copper foil having a wave-shaped cross-section ofReference Example C1.

Example N7

An electrode was produced as in Example N1 except that a flatelectrolytic copper foil current collector having a thickness of 12 μm(not coated with a milled carbon fiber layer) was used in place of thecopper foil having a wave-shaped cross-section of Reference Example C1.

Reference Example N1

An electrode was produced as in Example N1 except that the powder ofComparative Example M1 was used in place of the powder of Example M1containing silicon as a main component.

Reference Example N2

An electrode was produced as in Example N1 except that the powder ofComparative Example M2 was used in place of the powder of Example M1containing silicon as a main component.

Comparative Example N1

An electrode was produced as in Example N1 except that in Example N3,the powder of Comparative Example M1 was used in place of the powder ofExample M1 containing silicon as a main component; no cellulosenanofiber or nano bubble water was used; and a not-to-be-treated flatelectrolytic copper foil having a thickness of 12 μm was used in placeof the current collector of Comparative Example C1.

[Evaluation of Electrochemical Lithiation Amount of Electrode Structure]

The electrochemical lithiation (lithium insertion) amount of theelectrode structure for anode of the energy storage device as a singleelectrode was evaluated in accordance with the following procedure.

The electrodes of Example N1, Example N2, Reference Example N1, andReference Example N2 were produced as working electrodes. A cell wasproduced by combining the metal lithium serving as a counter electrodewith the produced electrodes, and the electrochemical lithiation amountwas evaluated. The lithium electrode was produced by crimping a metallithium foil having a thickness of 140 μm to an expand metal formed of anickel foil and punching the foil into a predetermined size. A pouchcell was used as an evaluation cell. The evaluation cell, which is apouch cell, was produced in accordance with the following procedure. Thepouch cell (laminate type cell) was produced in a dry atmosphere inwhich water having a dew point of −50° C. or less was managed. Anelectrode group consisting of working electrode/separator/lithiumelectrode was inserted into a battery jar obtained by forming analuminum laminate film having a polyethylene/aluminum foil/nylonstructure into a pocket shape; an electrolyte solution was injected; anelectrode lead was extracted; and the battery jar was heat-sealed toproduce an evaluation cell. A nylon film was formed outside the aluminumlaminate film, and a polyethylene film was formed inside the same. Usedas the separator was a microporous polyethylene film having a thicknessof 17 μm and a porosity of 40%. Used as the electrolyte solution was asolution obtained by dissolving 1M (mol/liter) of lithiumhexafluorophosphate (LiPF₆) in a solvent obtained by mixing sufficientlydehydrated ethylene carbonate and diethyl carbonate at a volume ratio of3:7.

The electrochemical lithiation amount was evaluated by discharging theproduced cell using the lithium electrode of the cell as an anode andeach working electrode as a cathode until the voltage of the cell became0.01 V and then charging the cell until the voltage became 1.80 V. Thatis, the amount of discharged electricity was defined as the amount ofelectricity used to store lithium, and the amount of charged electricitywas defined as the amount of electricity used to release lithium.

Charges and discharges were performed at a constant current on the orderof 0.2 C, and the coulombic efficiency of the first delithiation amount(the amount of electricity) corresponding to the first lithiation amount(the amount of electricity) and the second delithiation amount (mAh/g)were evaluated. The evaluation results are as follows.

Both the descending orders of coulombic efficiency and delithiationamount of the electrodes were as follows: Example N1>ExampleN2>Reference Example N1>Reference Example N2. These orders were the sameas the ascending order of the silicon oxide content of the activematerial containing silicon as a main component. The capacities of theelectrodes with respect to the weights of the active materials were all2500 mAh/g or more, and the coulombic efficiencies of the electrodes ofExample N1 and Example N2 were 90% or more.

The evaluation results indicate that the active material particlescoated with the lithium-containing composite oxide and containingsilicon as a main component of the present invention were a materialhaving a lower irreversible capacity in an electrochemical lithiuminsertion/extraction reaction.

[Production of Cathode]

Reference Example P1

One hundred parts by weight of nickel cobalt manganese lithium oxidepowder LiNi_(1/3)CO_(1/3)Mn_(1/3)O₂, 2 parts by weight of the flakegraphite prepared in Reference Example G1, and 2 parts by weight ofacetylene black were mixed. Fifty parts by weight of anN-methyl-2-pyrrolidone solution containing 10% by weight ofpolyvinylidene fluoride and 50 parts by weight of N-methyl-2-pyrrolidonewere added and kneaded to prepare a slurry for forming an electrodeactive material layer. Then, the resulting slurry was applied onto analuminum foil having a thickness of 14 μm using a coater, then dried at110° C. for 1 h, and further dried under a reduced pressure at 150° C.Then, the thickness was adjusted using a roll press to give an electrodestructure in which an electrode active material layer having a thicknessof 82 μm and a density of 3.2 g/cm³ is formed on a current collectorformed of a copper foil. The resulting electrode structure was punchedinto a predetermined size, and a nickel lead was welded to the tab ofthe aluminum current collector by ultrasonic welding to produce acathode electrode.

Reference Example P2

Lithium nitrate and nickel nitrate were mixed at a molar ratio of 1:5and dissolved in ethanol. To the resulting ethanol solution was added asmall amount of polyethylene glycol. Nickel cobalt manganese lithiumoxide powder LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂ was dispersed in the ethanolsolution. The dispersion solvent was passed through micropores formed bya pressuring diamond disc using an extreme pressure pump. The nickelcobalt manganese lithium oxide particles were crushed using the highlocal impact of cavitation which occurred during the passage, dried at100° C. using a spray dryer, and heat-treated at 600° C. to give nickelcobalt manganese lithium oxide particles coated with a lithium-aluminumcomposite oxide layer. Then, an electrode serving as a cathode wasproduced in a manner similar to Reference Example P1 above.

[reparation of Electrolyte Solution]

Reference Example E1

An electrolyte solution was prepared by dissolving 1M (mol/liter) oflithium hexafluorophosphate (LiPF₆) in a solvent obtained by mixingsufficiently dehydrated ethylene carbonate and diethyl carbonate at avolume ratio of 3:7.

Reference Example E2

An electrolyte solution was prepared by adding 2% by weight of2-vinylnaphthalene and 1% by weight ethylene glycol dimethacrylate tothe electrolyte solution of Reference Example E1.

Reference Example E3

An electrolyte solution was prepared by adding 0.05% by weight oflithium nitrate and 1.5% by weight of aluminum iodide to the electrolytesolution of Reference Example E1.

Reference Example E4

An electrolyte solution was prepared by dispersing 25% by weight of asolid electrolyte Li_(1.5)A_(10.3)Ti_(1.7)Si_(0.2)P_(2.8)O₁₂ which waspulverized into an average particle size of 100 nm or less and had aNASICON crystal structure, in the electrolyte solution of ReferenceExample E1.

[roduction of Energy Storage Device]

Example B1

A pouch cell (laminate type) lithium-ion rechargeable battery having astructure in which an anode having anode layers disposed on both sidesof an anode current collector is sandwiched between separator films andcathodes each having a cathode layer disposed on one side of a cathodecurrent collector (cathode/separator/anode/separator/cathode) wasproduced as an energy storage device. The use of the electrode havingthe electrode layers disposed on both sides of the anode currentcollector is intended to allow both sides of the current collector toapproximately equally receive stress which occurs due to the expansionof the volume during lithiation.

The pouch cell was produced using Example N1 as an anode and ReferenceExample P1 as a cathode. Also, the pouch cell was produced in a dryatmosphere in which water having a dew point of −50° C. or less wasmanaged. An electrode group consisting ofcathode/separator/anode/separator/cathode was inserted into a batteryjar obtained by forming an aluminum laminate film having apolyethylene/aluminum foil/nylon structure into a pocket shape; theelectrolyte solution of Reference Example E1 was injected; an electrodelead was extracted; and the battery jar was heat-sealed to produce acell for evaluating cathode capacity regulation. A nylon film was formedoutside the aluminum laminate film, and a polyethylene film was formedinside the same. Microporous polyethylene films having a thickness of 17μm and a porosity of 40% were used as the separators.

Example B2

A rechargeable battery was produced as in Example B1 except that theelectrode of Example N2 was used as an anode in place of the electrodeof Example N1.

Example B3

A rechargeable battery was produced as in Example B1 except that anelectrode of N3 using graphite powder having a particle size of 5 μm inplace of flake graphite serving as a conductive additive was used as ananode in place of the electrode of Example N1.

Example B4

A rechargeable battery was produced as in Example B1 except that anelectrode of Example N4 produced without mixing a cellulose nanofiberwas used as an anode in place of the electrode of Example N1.

Example B5

A rechargeable battery was produced as in Example B1 except that anelectrode of Example N5 produced without adding nano bubble water wasused as an anode in place of the electrode of Example N1.

Example B6

A rechargeable battery was produced as in Example B1 except that theelectrode of Example N6 using the current collector coated with themilled carbon fiber was used as an anode in place of the electrode ofExample N1.

Example B7

A rechargeable battery was produced as in Example B1 except that theelectrode of Example N7 using the flat electrolytic copper foil having athickness of 12 μm as a current collector was used as an anode in placeof the electrode of Example N1.

Example B8

A rechargeable battery was produced as in Example B1 except thatReference Example E2 containing vinyl monomer as an additive was used asan electrolyte solution; and the electrode of Reference Example P2 wasused as a cathode.

Example B9

A rechargeable battery was produced as in Example B1 except thatReference Example E2 containing vinyl monomer as an additive was used asan electrolyte solution.

Example B10

A rechargeable battery was produced as in Example B1 except thatReference Example E3 containing the inorganic additive was used as anelectrolyte solution.

Example B11

A rechargeable battery was produced as in Example B1 except thatReference Example E4 having the solid electrolyte dispersed therein wasused as an electrolyte solution.

Example B12

A rechargeable battery was produced as in Example B1 except that theelectrode of Reference Example P2 was used as a cathode.

Reference Example B1

A rechargeable battery was produced as in Example B1 except that theelectrode of Reference Example N1 was used as an anode in place of theelectrode of Example N1.

Reference Example B2

A rechargeable battery was produced as in Example B1 except that theelectrode of Reference Example N2 was used as an anode in place of theelectrode of Example N1.

Comparative Example B1

A rechargeable battery was produced as in Example B1 except that theelectrode of Comparative Example N1 was used as an anode in place of theelectrode of Example N1.

[Evaluation of Charge/Discharge Test]

The energy storage devices were charged at a constant current density of1 C, which is a condition in which an SEI layer is more likely to beformed due to the decomposition of the electrolyte solution and thecycle life is more likely to be reduced, until the cell voltage became4.4 V, then rested for 10 min, then discharged at a constant currentdensity of 1 C until the cell voltage became 2.5 V, and then rested for10 min. Such charge and discharge were repeated 100 times and then theenergy storage devices were evaluated for charge/discharge. As theevaluation results, the descending order of the amount of the 100thcharge/discharge was as follows: Example B8>Example B9>ExampleB10>Example B11>Example B12>Example B1>Example B6>Example B2>ExampleB4>Example B5>Example B7>Example B3>Reference Example B1>ReferenceExample B2>Comparative Example B1. The evaluation results indicate thatthe electrodes of the electrode structures of the present invention andthe energy storage devices of the present invention exhibited highperformance in terms of both the amount of charge/discharge andcharge/discharge repetition characteristics.

INDUSTRIAL APPLICABILITY

As described above, the present invention can provide ahigh-output-density, high-energy-density, and long-cycle-life energystorage device, an electrode structure for the anode of the energystorage device, and an active material (anode material) used in theelectrode structure for the anode.

DESCRIPTION OF REFERENCE SIGNS

-   101, 103 particle containing silicon as a main component-   102, 104 lithium composite metal oxide-   200 current collector-   201 active material-   202 flake graphite-   203 binder-   204 nanofiber-   205 electrode layer-   206 electrode structure-   301, 302, 303 metal foil-   304 binder-   305 milled carbon fiber-   306 layer containing milled carbon and binder-   401, 501, 601, 701 anode current collector-   402, 502, 602, 702 anode active material layer-   403 anode-   404, 503, 603, 703 ion conductor-   405, 504, 604, 704 cathode current collector-   406, 505, 605, 705 cathode active material layer-   407 cathode-   408, 608 battery jar (housing, package)-   409, 708 anode lead-   410, 506, 606, 706 anode terminal-   411, 709 cathode lead-   412, 507, 607, 707 cathode terminal-   508, 713 gasket-   710 safety valve-   711, 712 insulating plate

The invention claimed is:
 1. A method for producing, an anode activematerial for power storage devices capable of electrochemicallyinserting and extracting lithium ions, the anode active material having:secondary particles, wherein the secondary particles are aggregates of10-300 nm primary particles containing silicon as a main component,wherein the primary particles each include, as a surface layer, acomposite metal oxide layer containing at least one or more metalelements selected from at least Al, Zr, Mg, Ca, and La and Li, themethod comprising: forming the surface layer by immersing particlescontaining the silicon as a main component in an alcohol solutionobtained by dissolving a nitrate or organic acid salt of the one or moremetal elements selected from at least Al, Zr, Mg, Ca, and La and Li, oran alkoxide and then heat-treating the particles.